Method and apparatus for improving dehumidification

ABSTRACT

Comfort temperature and humidity levels in a conditioned space are controlled by cycling the speed of an evaporator fan motor in such a manner that, during time periods when higher sensible capacity is desired, the time in which the fan motor operates at a higher speed is increased, and during time periods when a higher latent capacity is desired, the time in which the fan motor operates at a lower speed is increased. The fan motor may be a single speed motor that is switched between on and off positions, or it may be a multiple speed motor that is cycled between a higher speed and a lower speed.

FIELD OF THE INVENTION

This invention relates generally to air conditioning systems and, more particularly, to control of an indoor fan to obtain improved dehumidification.

BACKGROUND OF THE INVENTION

In air conditioning systems, it is known that the gross cooling capacity of the refrigerant system, or the overall cooling capacity of an evaporator, is increased with indoor fan speed. This higher capacity is the result of higher indoor airflow moving over the evaporator external surfaces and therefore improving overall evaporator heat transfer coefficient.

It is also known that, when an evaporator fan is operating at a lower speed, with less air being passed over the evaporator, evaporator operates at a lower sensible heat ratio. In other words, the latent capacity component in the overall evaporator capacity is increased while the sensible capacity component is reduced, such that more moisture will be removed from each unit of the air on a relative basis and the overall relative humidity will be reduced.

There are many techniques to provide the desired levels of temperature and relative humidity comfort in conditioned spaces or climate-controlled zones. However, such techniques are quite often very complicated, involve additional expensive components and require complex control logic.

What is needed is a simple and inexpensive approach to controlling the temperature and relative humidity in a climate-controlled space by a conventional air conditioning system comprised of standard components.

DISCLOSURE OF THE INVENTION

Briefly, in accordance with one aspect of the invention, the respective levels of temperature and humidity in a conditioned space are controlled by selectively operating an evaporator fan by periodically switching between the first speed and the second speed, with the second speed being higher than the first speed. Furthermore, when higher cooling capacities are desired, the period of time associated with the indoor fan operation at the second speed is increased, and when lower relative humidity levels are desired, the period of time associated with the indoor fan operation in the first speed is increased.

For a multi-speed evaporator fan motor, the first speed is one of the lower available speeds and the second speed is one of the higher available speeds. For a single speed evaporator fan motor, the first speed is zero and the second speed is the only operational speed of that evaporator fan motor.

The frequency of switching between the first speed and the second speed may be determined based on reliability considerations and variations of temperature and humidity in the conditioned space.

In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an air conditioning system with the present invention incorporated therein.

FIG. 2 is a graphic illustration of the manner in which the evaporator fan motor is controlled in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is shown generally at 10 as incorporated into an air conditioning system 11 which includes, in serial flow relationship, a compressor 12, a heat rejection heat exchanger 13, an expansion device 14 and a heat accepting heat exchanger 16. The heat rejection heat exchanger assembly includes a motor driven air-moving device or fan 17 and the heat accepting heat exchanger assembly includes an air-moving device or fan 18 driven by an electric motor 15. As known, the heat rejection heat exchanger 13 is a condenser for subcritical applications and a gas cooler for transcritical applications, while a heat accepting heat exchanger 16 is generally known as an evaporator. In operation, the basic conventional air conditioning system 11 is typically utilized to provide cooling to a conditioned space 19. While the indoor airflow driven by the indoor fan 18 passes over external surfaces of the evaporator 16 and is delivered to the conditioned space 19, it is cooled and typically dehumidified, prior to entering the conditioned space 19. In the cooling mode of operation, the focus is typically on providing cooling to the conditioned space 19, while dehumidification occurs as a side effect of this cooling process. In many cases, it is desired to shift the focus from cooling to dehumidification, or in other words, rebalance sensible and latent capacity components in the overall capacity provided by the air conditioning system 11. Therefore, the operation of the evaporator fan motor 15 is controlled in a manner which allows for selectively controlling the humidity level in the conditioned space 19 when desired.

Within the conditioned space 19, there is provided a temperature sensor 21 and a humidity sensor 22 whose sensed values are sent along the respective transmission lines 23 and 24 to a control 26. The control 26 then responsively operates a relay 27 to control the speed of the evaporator fan motor 15 in a manner to be described hereinafter. In this regard, it should be recognized that the relay 27 is only representative of various other types of switching devices that can be used for this purpose. For example, an electronic switch or an optical switch could also be used in place of the relay 27.

It is recognized that, during periods of high demand for cooling such as high ambient temperature conditions or high heating thermal load in the conditioned space 19, there will be a need for higher cooling capacity of the air conditioning system 11 and accordingly higher sensible cooling performance of the evaporator 16. In order to obtain that higher sensible cooling, the evaporator fan motor 15 is operated at full speed for a single speed motor or at the highest speed for a multi-speed (e.g. two-speed) motor, so as to obtain the maximum amount of air being passed over the external surfaces of the evaporator 16. During this mode of operation, the increased evaporator airflow and elevated saturation suction temperature of the refrigerant will provide maximum evaporator capacity, but on a relative basis, its latent component will be reduced such that the relative humidity in the space 19 may tend to increase. Since in this mode of operation, the focus is on sensible cooling, the humidity sensor feedback potentially showing slight relative humidity increase will be overridden.

On the other hand, during other periods of operation, the heat load in the conditioned space 19 may be at a moderate level or environmental conditions may not be that severe (relatively low ambient temperatures). In these cases, the full cooling capacity provided by the air conditioning system 11 and full sensible capacity of the evaporator 16 are not required. Thus, during these operational periods of the reduced cooling demands, the system may be controlled to operate with the focus on dehumidification to primarily reduce the relative humidity in the conditioned space 19. To accomplish this task, the evaporator fan motor 15 is switched between the first lower speed and the second higher speed, in order to obtain a reduced overall time-averaged speed. At a reduced time-averaged speed of the evaporator fan motor 15, a lower time-averaged airflow is passing over external surfaces of the evaporator 16 resulting in a lower refrigerant saturation suction temperature. As known, at lower saturation suction temperatures (or suction pressures), the latent component of the evaporator capacity is increased, on a relative basis, which, in turn, tends to reduce the relative humidity in the conditioned space 19. At the same time, the reduced time-averaged airflow causes sensible component of the evaporator capacity to decrease, which is acceptable since the higher cooling capacity is not required during this time periods of reduced cooling demands. The two phenomena mentioned above, of course, promote lower sensible heat ratios (SHR) as desired.

For a multi-speed evaporator fan motor, the first speed is one of the lower available speeds and the second speed is one of the higher available speeds. The switching may be performed between any of the lower speeds and any of the higher speeds. For a single speed evaporator fan motor, the first speed is zero and the second speed is the only operational speed of that evaporator fan motor.

The frequency of switching between the first speed and the second speed may be determined based at least on reliability considerations and variations of temperature and humidity in the conditioned space. On one hand, too frequent switching may reduce reliability of the air management components of the air conditioning system 11. On the other hand, prolonged operational time intervals at each speed may cause excessive temperature and humidity fluctuations in the conditioned space 19. The time spent at each speed level within one cycle determines time-averaged speed for the evaporator fan motor 15 and time-averaged sensible and latent components of the capacity for the evaporator 16.

It has to be pointed out that, if the switching technique described above is implemented to a single speed motor, the evaporator fan 15 does not have to be brought to a complete stop and may be engaged again in a consequent cycle while still rotating due to inertia that would assist in a starting torque and power consumption reduction in a subsequent cycle. Further, the evaporator fan motor 15 may be engaged and disengaged by shutting off electric power provided to the evaporator fan motor 15 or an evaporator fan 18 may be engaged and disengaged by mechanical means such as an electro-magnetic clutch.

Further, it has to be understood that air conditioning system 11 depicted in FIG. 1 is a very basic system and shown for illustrative purposes only. In reality, air conditioning system 11 may have many enhancement features and design options. All such systems are within the scope and can equally benefit from the invention. For instance, to further control capacity, the air conditioning system 11 may have various unloading options known in the art, such as hot gas bypass, suction modulation, multiple circuits, vapor injection, economizer-to-suction bypass, digital scroll compressor, two-stage compressor, cylinder unloading for a reciprocating compressor, etc. All these unloading options can be used in combination with indoor fan cycling to control latent and sensible components of the capacity for the evaporator 16.

Referring now to FIG. 2, there is shown a graphic illustration of the method in which the evaporator fan motor 15 is controlled in a simple and efficient manner to obtain the speed changes corresponding to required thermal load demands in the conditioned space 19 as described hereinabove.

The evaporator fan motor 15 may be a single speed motor or a multiple speed motor (e.g. two speed motor). As explained above, if the evaporator fan motor 15 is a single speed motor, the motor is cycled alternately between the on and off positions as shown in FIG. 2 wherein:

-   -   T—time interval of a single cycle     -   t₁—time interval when the evaporator fan is off or disengaged     -   t₂—time interval when the evaporator fan is on or engaged

t ₁ +t ₂ =T

Alternatively, if the evaporator fan motor 15 is a multi-speed motor, it is selectively cycled between one of the higher available speeds and one of the lower available speeds as shown in FIG. 2. In this case, the values T, t₁ and t₂ have the following meaning:

-   -   T—time interval of a single cycle     -   t₁—time interval when the evaporator fan is operating at a lower         speed     -   t₂—time interval when the evaporator fan is operating at a         higher speed

t ₁ +t ₂ =T

In both cases time-averaged speed is defined as follows:

S=(S _(i) *t _(i) +S ₂ *t ₂)/T

-   -   S₁—a lower operational speed     -   S₂—a higher operational speed

The values of T, t₁ and t₂ can be selectively varied as desired to accomplish the desired result of achieving latent and sensible components of the evaporator capacity while satisfying reliability requirements as well as temperature and humidity variations in the conditioned space. That is, the time ratio of t₁/t₂ can vary from 0 to ∞. For instance, in a variety of applications, the period of a single cycle, T, can be varied within a range of 5 second to 2 minutes. As mentioned above, the cycle frequency is determined by the reliability requirements in order not to overheat or wear cycling components as well as limitations imposed on variations of temperature and humidity in the conditioned space, while the time at the higher speed t₂ is defined by the latent and sensible components of the evaporator capacity required to satisfy thermal load demands in the conditioned space.

Since an air conditioning system and an associated conditioned space have significantly large thermal inertia, such evaporator fan cycling and airflow fluctuations should not affect environmental parameters in the climate-controlled space. Further, the provided time-averaged airflow should not deviate much from the mean value preventing uncomfortable conditions for an occupant of the conditioned space. Since the air duct system has significant volume, such airflow fluctuations will be reduced well before reaching the conditioned environment.

As known in the art, extremely low airflows may cause evaporator surface temperature to fall below a freezing point. Therefore, conventional methods of control, such as low suction pressure cutoff or low saturation suction temperature threshold, may be utilized to prevent evaporator frosting conditions.

It should be recognized that, the present invention is applicable to all types of air conditioning systems such as commercial and residential air conditioning and heat pump systems as well as mobile applications. Also, no new hardware is required to implement the invention.

While the present invention has been particularly shown and described with reference to a preferred embodiment as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims. 

1. A method of controlling the humidity and temperature in a conditioned space comprising the steps of: providing an air conditioning system having an evaporator and an associated evaporator fan motor; and selectively operating said fan motor by rapidly cycling between a first speed and a second speed, with said second speed being higher then said first speed, to change the latent or sensible capacity.
 2. A method as set forth in claim 1 and further wherein, when a higher sensible capacity is desired, the time in which the fan motor operates at said second speed is increased, and when a higher latent capacity is desired, the time in which the fan motor operates at said first speed is increased.
 3. A method as set forth in claim 1 wherein said evaporator fan motor is a single speed motor and further wherein said first speed comprises an “off” disengaged condition and said second speed comprises an “on” engaged condition.
 4. A method as set forth in claim 3 wherein said evaporator fan motor is brought to a complete stop before starting the next consecutive cycle.
 5. A method as set forth in claim 3 wherein said evaporator fan motor is not brought to a complete stop before starting the next consecutive cycle.
 6. A method as set forth in claim 3 wherein said evaporator fan motor is electrically engaged and disengaged.
 7. A method as set forth in claim 3 wherein an associated evaporator fan is mechanically engaged and disengaged from said evaporator fan motor.
 8. A method as set forth in claim 7 wherein the engagement and disengagement mechanism is a clutch.
 9. A method as set forth in claim 1 wherein said evaporator fan motor is a multiple speed motor and further wherein said first speed comprises a lower speed from an available set of speeds for the fan motor and said second speed comprises a higher speed from an available set of speeds for the fan motor.
 10. A method as set forth in claim 9 wherein said multiple speed motor comprises a dual speed fan motor.
 11. A method as set forth in claim 1 wherein said step of selectively operating said evaporator fan motor is accomplished by way of a relay, a contactor, an electro-magnetic switch or an optical switch.
 12. A method as set forth in claim 1 wherein the fan motor cycling frequency is defined by at least one of reliability considerations for cycled components, temperature variations in a conditioned space, humidity variations in a conditioned space, airflow fluctuations, and evaporator freeze conditions.
 13. A method as set forth in claim 1 wherein the time interval at which fan operates at said higher speed is defined by at least one of the latent capacity requirement and sensible capacity requirement.
 14. A method as set forth in claim 1 wherein the time of a full cycle of operation for said fan motor comprised of the time of operation at said first speed plus the subsequent time of operation at said second speed is in the range of 5 seconds to 2 minutes.
 15. Apparatus for controlling the humidity and temperature in a conditioned space, comprising: an air conditioning system having an evaporator and an associated evaporator fan motor; a switch capable for rapidly switching said fan motor between a first speed and a second speed; and a controller for selectively operating said switch by rapidly cycling between said first speed and said second speed, with the second speed being higher then the first speed, to change the latent or sensible capacity.
 16. Apparatus as set forth in claim 15 wherein said controller operates said switch in such a manner that, when a higher sensible capacity is desired, the time in which the fan motor operates at said second speed is increased, and when a higher latent capacity is desired, the time in which the fan motor operates at said first speed is increased.
 17. Apparatus as set forth in claim 16 wherein said evaporator fan motor comprises a single speed motor and further wherein said first speed comprises an “off” disengaged condition and said second speed comprises an “on” engaged condition.
 18. Apparatus as set forth in claim 17 wherein said evaporator fan motor is brought to a complete stop before starting the next consecutive cycle.
 19. Apparatus as set forth in claim 17 wherein the evaporator fan motor is not brought to a complete stop before starting the next consecutive cycle.
 20. Apparatus as set froth in claim 17 wherein said evaporator fan motor is electrically engaged and disengaged.
 21. Apparatus as set forth in claim 17 wherein an associated evaporator fan is mechanically engaged and disengaged from said evaporator fan motor.
 22. Apparatus as set forth in claim 21 wherein the engagement and disengagement mechanism is a clutch.
 23. Apparatus as set forth in claim 15 wherein said evaporator fan motor comprises a multiple speed motor and further wherein said first speed comprises a lower speed from an available set of speeds for the fan motor and said second speed comprises a higher speed from an available set of speeds for the fan motor.
 24. Apparatus as set forth in claim 23 wherein said multiple speed motor comprises a dual speed motor.
 25. Apparatus as set forth in claim 15 wherein said switch comprises a relay, a contactor, an electro-magnetic switch or an optical switch.
 26. Apparatus as set forth in claim 1 wherein the fan motor cycling frequency is defined by at least one of reliability considerations for cycled components, temperature variations in a conditioned space, humidity variations in a conditioned space, airflow fluctuations, and evaporator freeze conditions.
 27. Apparatus as set forth in claim 1 wherein the time interval at which fan operates at said higher speed is defined by at least one of the latent capacity requirement and sensible capacity requirement.
 28. Apparatus as set forth in claim 1 wherein the time of a full cycle of operation for said fan motor comprised of the time of operation at said first speed plus the subsequent time of operation at said second speed is in the range of 5 seconds to 2 minutes. 